Another idea would be integrate with SmartThings since they also have multi sensors for temperature, humidity, etc.
I hope the Iro will also be able to use the default weather channel on IFTTT since I don't have a Netatmo or SmartThings at the moment.
I would suggest the Parrott moisture device but I haven't read great things about it.
You could have something like a single or group of Phillips Hue bulbs blink, change colors, etc when a specific zone and/or watering time is activated. Then also an action that when a zone and/or watering time is finished, then the Phillips Hue bulb or group of bulbs blink, change colors, etc.
I think that whatever you can use to notify when a zone and/or watering time is finished, that might be cool. Even if it's an ios notification - I don't believe that's possible today, but I could be wrong.
I'll keep trying to think of things. Perhaps an ongoing list of triggers and actions can be added somewhere here so we know what is going to be possible, what isn't and what is currently live or created.